Arrangement for the background noiseless production of sound films



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ARRANGEMENT FOR THE BACKGROUND NOISELESS PRODUCTION OF SOUND FILMS Filed Dec. 13, 1939 2 Sheets-Sheet 1 Fig.1

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ARRANGEMENT FOR THE BACKGROUND NOISELESS PRODUCTION OF SOUND FILMS Filed pee. 13, 1939 2 Sheets-Sheet 2 RQM A 5601 ega- Patented Sept. 29, 1942 ARRANGEMENT FOR THE BACKGROUND NOISELESS PRODUCTION OF SOUND FILMS Walter Diille'nbach, Berlin, Germany; vested in the Alien Property Custodian Application December 13, 1939, Serial No. 309,111 In Germany December 19, 1938 4 Claims.

- trical circuits leading directly to the recording device, to direct the energy through a rectifier,

' and then to feed it back to the recording device together with the vibrations of speech and sound. Consequently when a great amplitude of the vibrations is recorded, the middle line is widely separated from the time axis, while at a small amplitude of vibration the middle line lies closely to the time axis, the practical efiect being that almost only the minima of the recorded vibration are impressed, independent of the vibration amplitude, on the time axis.

The system according to this invention for reducing a disturbance level, especially for the controlling of recording devices to eliminate the background noise, preferably for non-power-con suming control of recording devices for the production of light-sound-films, for instance for the controlling of Brauns tubes, is characterized in that all of the sound energy, converted into electrical energy, which serves for controlling a recording element, is directed through a circuit consisting of at least one rectifier and a condenser connected in series, the condenser being in parallel with a resistance, and further, that the electrical energy serving for the control of the recording device, is taken from two points situated on different sides of the rectifier.

The system according to the invention may be mounted into the recording] apparatus at any desired point, for instance between themicrophone and pre-amplifier, between the pre-amplifier and main amplifier, or between the endamplifier and recording device. In particular the system according to the invention may also be developed as an auxiliary apparatus, for instance in the form of an intermediate plug-and-' socket, and joined to existing apparatus working without reduction of the back-ground noise. The invention is especially applicable to portable apparatus and amatur-narroW-film-picturesound apparatus.

Other features and advantages of the inven- '(light controlling device, Brauns tube).

tion will appear from the following description, taken in connection with the accompanying drawings, in which,

Fig. 1 is a view of the principal electrical circuit;

Fig. 2 illustrates diagrammatically and in gen-' eral, the relation between an alternating voltage and the resulting controlling voltage;

Fig. 3 is a'diagrammatic view of the charge voltage of condenser 6 bridged by a resistance;

Fig. 4 illustrates the operation of a system according 'to the invention;

Fig.5 relates to a controlling circuit in accordance with the invention, and

Fig, 6 illustrates diagrammatically the operation of a circuit as represented in Fig. 5.

Referring to 'Fig. 1, the vibrations to be registered are supplied to the system at the terminals 4. A rectifier 5 and a condenser 6 are connected in series and to the terminals. The energy to be recorded is taken from the terminals l8 and supplied to the recording device The condenser 6 is bridged by a resistance III, which may be controllable. The rectifier 5 can be of any type with a suitably bent current-voltagecurve, for instance an electron tube, especially a high-vacuum incandescent diode, or a dry-rectifier.

The mode of operation of this arrangement is as follows; I

By feeding the system with a pure alternating' voltage, the condenser 6 is charged to the amplitude of the alternating voltage prevailing between the terminals 4. These conditions are schematically represented in Figure 2. The. time axis is represented by I. The alternating voltage prevailing between the terminals 4 is indicated by I. The voltage in the condenser 6 being' essentially constant is represented by the line 8. I

Then the voltage at the terminals [8 which serves to control the registration device, is composed of the direct voltage determined by rectifier 5 and condenser 6, and of the varying soundvoltage, 'being impressed on it. The effective controlling voltage, therefore, can be'represented as a difierential voltage between line 8 and curve 1, as shown by the arrows 8 In condenser 6 isbridged by a, high resistance, the conditions shown in Figure 2 and characterized by constant condenservoltage, are replaced by a varying condenser voltage which is represented by line I l in Figure 3. Suppose the input energy impressed upon the terminals 4 to be.

is marked ll. From this point of intersection the input energy impressed upon the terminals 4 will determine the progress of the condenser .yoltage and raise it back to the apex value, from which another lineardrop occurs. This will be continued in' a regular sequence, from one half wave to another.

The voltage which now prevails at the rectifier element 5 is no longer in exact agreement with the alternating voltage I on which a direct voltage is impressed, but has a noise factor, relative to the really desired voltage, conditioned by the fact that the condenser voltage drops as shown by curve H. However, the invention contemplates that resistance H) has a comparatively high value and the condenser voltage drops relatively slowly. In the case of sound frequency, for instance the voltage-drop occupies a time interval of irom 1/4 to 1 second which makes the noise factor completely negligible.

In describing the mode of operation of the system as shown in Figures 2 and 3, it has been assumed the amplitude of the alternating voltage I to be constant. In contradistinction, in Fig. ure 4, the mode of operation is shown when the amplitude of the alternating voltage is changed,

'this being the case in which the intended effect of noise-reduction is obtained. The alternating voltage is marked by [2, I being the time axis. Between points l3 and H, the amplitude of the alternating voltage drops froma high amplitude l5 to a much smaller amplitude It. When this drop takes place more quickly than the drop in the condenser voltage curve H, the latter will deviate somewhat from a curve enveloping the alternating voltage curve l2. These conditions are depicted in Figure 4, between the amplitudes i2 and I2". However, as soon as the amplitudes of .curve l2 become nearly constant again, the condenser voltage curve IT will return to the peaks of curve l2 and supply the terminals I8, via the rectifier element 5, with a voltage adequate to produce recording free from back-ground-noise.

Figure 4 clearly shows the operation of a controlling device according to the invention; for, as the condenser voltage [1 is the direct voltage upon which an alternating voltage is impressed, it plainly follows that the shifting of curve I1 according to Fig. 4 will efiect a shifting of the middle line in photographic sound recording.

The rectifier system shown in'Figure 1 may be replaced by any other rectifier circuit, as for example a transformer connection which is featured by two rectifier elements working in a socalled doubleway-connection, or Graetz connection, or any other rectifying system, the essence always being that the primary alternating voltage with a fixed middle line is supplied to a circuit comprising at least one rectifier element and one condenser connected in series, and that there is, in parallel to that condenser, a circuit similar to a leak resistance, and further that the alternating voltage supplied to the recording device is taken from the terminals of the rectifier element.

In a modification of the arrangement shown in Figure l, the resistance l0 is made variable, in particular by substituting for it an electron. tube, the internal resistance of which dependsupon the speed of amplitude change. It is advantageous to give this resistance the highestgpossible value as long as there is an increase in the amplitude of the vibrations to be recorded. On the other hand, when the amplitude decreases, the value of the resistance has to be made smaller at the same rate at which the amplitude decreases. By thus controlling the resistance to, Fig. 4, a curve tangent to the peaks of curve [2, and curve I! will deviate from one another by negligible amounts only, even though the amplitude of the vibration to be recorded, may change comparatively quickly.

Figure 5 shows a circuit having a, controlled variable resistance inparallel with the' condenser 6, as mentioned before. The vibration energy is supplied to the terminals 4, and taken ,at the terminals [8, as in Figural. The rectifier is represented by an incandescent diode 5, as shown in Figure 5. Number 6 again indicates the condenser connected in parallel to a resistance which in this special case is an electron tube 20. In parallel with the elements 5 and 6 is a similar circuit comprising the rectifier tube 25, the condenser 26 and the resistance 30 which has a comparatively small value. From point 32 between tube 25 and the condenser 26, a conductor 24 communicates with the grid of tube 20.

The operation of the circuit shown in Figure 5 may be explained in connection with Figure 6. I again represents the time axis and 2! stands for the oscillations to be recorded which, at 28, suddenly become much smaller than they were before. If there were no control for the resistance bridging the condenser 6, the direct voltage supplied by the condenser 6 and rectifier 5 would be maintained as shown by curve-29,that is it would fall very slowly, even though the amplitude of the sound waves 21 might decrease abruptly at it does at 28 in Figurefi. As the resistance 30 is very weak, the direct voltage supplied by the rectifier 25, the condenser 26 and the resistance 30 has entirely different characteristics, that is it readily adapts itself to the quick change of amplitude, as indicated by the dotted curve 3|. As appears from Figure 6, a potential difference between points 32 and 33 in Figure 5 occurs at time 28, and will cause a change of the grid bias of tube 20 and thereby, of its internal resistance in such a manner, that the voltage curve 29 decreases at'28 as indicated by curve 29'. To give the grid of tub 20a constant bias (static bias) ;battery 21 is provided together with a high resistance 23. To avoid an alternation of grid bias induced by the sound wave voltage component between the cathode and grid of tube 20 and following their rhythm, a condenser 22 is also provided.

As mentioned above, the invention may be practiced most advantageously in' cases where a light controlled element needing no controlling ene gy, such as the controlling electrode of a Brauns tube, is connected to the terminals I8, Figure 1. The reason is, that, if energy is consumed at the terminals l8 to any essential extent, a fiow of current will take place through the condenser 6 with resultant voltage drops. If such voltage drops occur they may be compensated somewhere in the course of the recording process in a known manner. This is preferably done with the aid of a resistance interposed in the rectifiercondenser circuit, provision being made for the resistance value to change as a function of the energy consumption which takes place in the apparatus connected to the output terminals. Such regulation may also be efiected automatically,

and the resistance may be of any suitable type;

A feature which deserves special mention lies in the fact that the invention is not only applicable to cases as described before in the process of sound wave recording, but can be used to advantage-wherever'the problem consists in lowering a disturbance level, such as reducing valve rustle in amplifiers, as in push-pull B amplifiers, or lowering the disturbance level in television amplifiers.

What I claim is:

1. An electrical system for modifying an alterterminal of the source of alternating current to be modified, means for taking the modified current from terminals connected to opposite sides. respectively, of said rectifier, and means for antomatically varying said resistance in response to variations in said source of current.

2. An electrical system as in 'claim 1, means for automatically varying said resistanc in respouse to the rate of change of the amplitude of said current to be modified.

3. An electrical system as in claim 1, said resistance comprising a triode vacuum tube, and means for controlling the grid bias of said tube in response to the rate of change of the amplitude of the current to be modified whereby said resistance is varied.

4. An electrical system as in claim 1, said resistance comprising a triode vacuum tube, a second system comprising a rectifier in series with a condenser coupled in parallel to a relatively low resistance, means coupling said second system in parallel with said first system, and means connecting the grid bia of said tube to said second system for control thereby.

WALTER DKLLENBACH. 

